The present invention relates to sensing a process variable of an industrial process. More specifically, the present invention relates to sensing a pressure of a process fluid used in such an industrial process.
Pressure sensors are used to measure pressures. One technique which is used to measure pressure is through a deflectable diaphragm. A pressure is applied to one side of the diaphragm to cause the diaphragm to deflect. The deflection of the diaphragm is proportional to the applied pressure. Diaphragm deflection can be measured by configuring the diaphragm so that it acts as a plate of a capacitor. The capacitance of the diaphragm assembly changes based upon the diaphragm position. Therefore, capacitance can be related to diaphragm position which in turn is representative of the pressure applied to the diaphragm.
Pressure sensors frequently operate in harsh environments. Such environments can cause failure in highly accurate pressure sensors which tend to be quite delicate. One such technique uses an isolation diaphragm in which process fluid is on one side of the isolation diaphragm and an isolation (or “fill”) fluid is on the other side. The isolation fluid contacts the pressure sensor diaphragm. As the process fluid pressure changes, the isolation diaphragm, responsively deflects which causes the change in pressure to be transferred through the isolation fluid to the sensor diaphragm. However, the isolation fluid in the diaphragm can introduce errors in pressure measurements and can be altered over time or due to application of heat or other external influences. Further, the fluid can leak with time which can result in reduced fluid volume or isolation fluid which has been contaminated by process fluid. Both a fluid leak or contamination cause errors in pressure measurement. Another example pressure sensing technique uses an elongate pressure sensor of a brittle material which is covered by a protective element such as that shown an described in U.S. Pat. No. 6,848,316, entitled PRESSURE SENSOR ASSEMBLY and assigned to Rosemount Inc.